This invention relates generally to drive systems for rotary devices, and more particularly to drive systems for rotary automotive accessory devices such as alternators.
Some systems which employ rotary prime movers or drivers for providing rotational motive power for driving rotary accessory devices are characterized by dynamic loading and inertial torque characteristics which result in rotational perturbations that are transmitted to the accessory devices. An example of such systems is an internal combustion engine that drives rotary automotive accessory devices such as an alternator, air-conditioning compressor, water pump, etc., in vehicles. Rotation of the engine crankshaft is transmitted via a serpentine poly-V belt system to pulleys attached to the drive shafts of such accessory devices to rotate their shafts. The rotation of an internal combustion engine crankshaft is, however, subject to perturbations, the magnitude and frequency of which varies with engine RPM. During combustion, the crankshaft temporarily speeds up and generates a pulse of rotational power that is transmitted via the belt to the rotary accessories. During compression, the crankshaft temporarily slows down. Thus, the crankshaft continually exhibits acceleration and deceleration and effectively imparts a pulsed driving characteristic to the serpentine belt system, which in turn is transmitted to the accessory devices. Generally, the slower the rotational speed of the crankshaft or the fewer the number of cylinders, the greater the pulse effect. At engine idle, for instance, the magnitude of the variations is the greatest and the effects most noticeable.
Crankshaft pulsations are transmitted to the serpentine belt system and the driving pulleys of accessory devices as dynamic rotational velocity fluctuations. However, the inertias of the rotary devices tend to resist the velocity fluctuations. This generates dynamic tensions in the belt as it tries to accelerate and decelerate the rotary devices to accommodate the fluctuations. These fluctuations are transmitted to the shafts of the rotary devices through their pulleys. Conventional belt tensioners cannot totally compensate for these dynamic fluctuations that may produce undesirable belt slippage, noise and vibration that are transmitted to the passenger compartment, as well as cause wear and tear on the rotary devices. This results in higher than desirable belt wear and shortens the life of the rotary devices. Automotive alternators are particularly susceptible to increased wear and decreased life due to such fluctuations because of their high inertia and high speed, and they tend to fail frequently.
One approach which has been proposed to address the problem of dynamic fluctuations and reduced life of rotary devices, such as automotive alternators, has been to employ one-way clutches in the pulleys of the rotary devices. Conventional one-way clutches are mechanical devices that engage when the alternator pulley rotates in the driving direction but disengage when the pulley rotates in the opposite direction relative to the shaft. One-way clutches accommodate crankshaft slowdown reasonably well since they overrun and disengage the pulley from the shaft and permit the shaft to continue rotating under the inertia of the alternator shaft and armature. However, one-way clutches do not accommodate well abrupt increases in speed, as when combustion occurs, since they engage suddenly and try to accelerate the shaft rotation rapidly to match the increased belt velocity. Repeated sudden engagement of the one-way clutch with the pulley results in high wear and frequent failure of the one-way clutch, and may shorten the life of the alternator bearings. Thus, one-way clutches used in high frequency loading environments have high failure rates. Moreover, one-way clutches do not eliminate the problems of rotational velocity fluctuation, noise and vibration since they address only belt deceleration but not belt acceleration.
Accordingly, another approach that has been proposed is to implement an isolator for an alternator pulley with a one-way clutch implemented using coil springs that permit some relative resilient rotational movements in opposite directions with respect to the alternator pulley. When the pulley accelerates, a coil spring about the shaft tightens and engages the shaft rapidly, typically in about a degree or so of angular rotation, to impart rotation to the shaft and another coil spring engages the shaft to permit some resilient relative rotation. In the opposite over-running direction, the pulley is free to decelerate relative to the alternator shaft.
There is a need for an improved drive system for coupling a rotating prime mover to the shaft of a rotating device that compensates for sudden relative rotational angular velocity differences between the prime mover driver and a driven device due to sudden acceleration and deceleration of the prime mover, by dampening and cushioning bidirectional relative rotations between the driver and the device. More particularly, it is desirable to provide an improved driving system for a rotary device in a dynamically changing environment that is simpler, more reliable, has a longer lifetime, and that affords better compensation of noise and vibrations than known drive systems. It is to these ends that the present invention is directed.